The present invention relates to an improved process for the electrochemical production of alkali metal from alkali metal amalgam. For the purposes of the present invention, the term xe2x80x9calkali metalxe2x80x9d refers to sodium and potassium.
The invention also relates to an electrolysis cell suitable for carrying out this process and to the principle of a production plant.
Sodium is an important basic inorganic product which is used, for example, for the preparation of sodium amide, sodium alkoxides and sodium borohydride. It is obtained industrially by the Downs process by electrolysis of molten sodium chloride. This process has a high energy consumption of xe2x89xa610 kWh/kg of sodium (Buchner et al., Industrielle Anorganische Chemie, 2nd edition, Verlag Chemie, p. 228 ff). The process also has the serious disadvantage that the electrolysis cells are destroyed by the solidification of the salt melt when they are switched off. Furthermore, the sodium metal obtained by the Downs process has the disadvantage that it is, as a result of the process, contaminated with calcium whose residual content can only be reduced, but never completely eliminated, by subsequent purification steps.
Potassium is likewise an important basic inorganic product which is used, for example, for producing potassium alkoxides, potassium amides and potassium alloys. It is nowadays obtained industrially primarily by reduction of potassium chloride with sodium. This process first produces NaK which is then fractionally distilled. A good yield is achieved by continually taking off potassium vapor from the reaction zone, thus shifting the equilibrium to the potassium side (Ullmann""s Encyclopedia of Industrial Chemistry, 6th edition 1998, Electronic Release). A disadvantage is that the process operates at high temperatures (870xc2x0 C.). In addition, the potassium produced contains about 1% of sodium and therefore has to be purified by a further rectification. The greatest disadvantage is that the sodium used is expensive. This is because, inter alia, sodium is obtained industrially by the Downs process by electrolysis of molten sodium chloride, which requires an energy consumption of at least 10 kWh/kg of sodium. This corresponds to about 5.3 kWh/kg of potassium (at 100% yield).
Sodium amalgam and potassium amalgam are intermediates which are formed in large quantities in chloralkali electrolysis by the amalgam process and are generally reacted with water immediately after they are produced to give an alkali metal hydroxide solution. The alkali metal amalgam which has been depleted in alkali metal or is now free of alkali metal is normally recirculated directly to the chloralkali electrolysis. In order to keep the sodium amalgam in liquid form, the sodium concentration has to be kept to values of less than 1% by weight, preferably from 0.2 to 0.5% by weight. To keep a potassium amalgam in liquid form, the potassium concentration has to be less than 1.5% by weight, preferably in the range from 0.3 to 0.6% by weight. The amalgams obtained on an industrial scale contain essentially metallic impurities in a concentration range from 1 to 30 ppm, for example copper, iron, potassium in sodium amalgam or sodium in potassium amalgam, lead and zinc.
GB 1,155,927 describes a process by means of which sodium metal can be extracted from sodium amalgam by an electrochemical route using a solid sodium ion conductor such as beta-Al2O3 with amalgam as anode and sodium as cathode. However, carrying out the process described in GB 1,155,927 does not lead to the results described there in respect of sodium conversion, product purity and current density. Furthermore, the system described displays unstable behavior over the course of a few days if the temperature range claimed is adhered to.
It is an object of the present invention to provide an improved process for the electrochemical production of alkali metal from an alkali metal amalgam, which process allows the production of sodium using less energy than the Downs process or allows the production of potassium using less energy than the industrial process discussed at the outset. For this purpose, the process described in GB 1,155,927 is to be decisively improved so that the new process can be integrated into the existing system of a chloralkali electrolysis by the amalgam method and the disadvantages found when carrying out the process of GB 1,155,927 are avoided.
The new process has to meet the following essential requirements:
The alkali metal conversion on the anode side has to satisfy the balance requirements of the product link with the chloralkali electrolysis. This means that the outflow concentration of alkali metal in the amalgam from the chloralkali electrolysis corresponds to the feed concentration in the alkali metal electrolysis of the present invention. Furthermore, the amounts of amalgam circulated between the chloralkali electrolysis and the alkali metal electrolysis of the present invention have to be kept within an order of magnitude which is technically and economically justifiable. In general, this is achieved if 50% of the alkali metal content of the feed amalgam is converted in the alkali metal electrolysis. The sodium metal has to be obtained directly in such a purity that further process steps to remove mercury can be omitted and the disadvantage of calcium contamination from which the Downs process suffers is avoided. The potassium metal has to be obtained directly in such a purity that further process steps to remove mercury can be omitted and the sodium content is less than that obtained in the reduction with sodium, where the potassium produced directly contains 1% of sodium. The process should be able to be carried out on an industrial scale and therefore has to make possible sufficiently high current densities and space-time yields. For reasons of physical structure of the production building, of safety, of environmental protection and of working capital, an equipment concept which makes do with a relatively small amount of mercury in the system is required. The process should be stable in long-term operation and tolerate without damage the usual metallic impurities occurring in industrial alkali metal amalgam. For the purposes of the present invention, xe2x80x9calkali metal amalgamxe2x80x9d refers to a solution of an alkali metal in mercury that is liquid at room temperature.
We have found that this object is achieved by the process of the present invention.
The present invention accordingly provides a process for producing alkali metal from alkali metal amalgam by electrolysis using an anode containing alkali metal amalgam, a solid electrolyte conducting alkali metal ions and a liquid alkali metal as cathode, wherein the alkali metal amalgam as anode is kept in motion.
In the process of the present invention, the anode is held at such a potential that only alkali metal is anodically oxidized to alkali metal ions, the ions are transported through the solid electrolyte in the electric field and are finally reduced cathodically to form alkali metal.
In addition, the present invention provides a specifically adapted electrolysis cell comprising a tubular solid electrolyte ({fraction (1/31)}) which is closed at one end and is installed in a concentric stainless steel tube (33) so as to form an annular gap. The process of the present invention can be particularly advantageously operated on an industrial scale in this electrolysis cell.